J. Phys. Chem. 1989, 93, 5549-5562
5549
Molecular Ionization and Dissociative Ionization at Hyperthermal Surface Scattering Albert Danon and Aviv Amirav* Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Ramat Aviv 69978 Tel Aviv, Israel (Received: September 15, 1988; In Final Form: December 27, 1988)
Surface ionization of molecules with hyperthermal kinetic energy (1-20 eV) was found to be very efficient, demonstrating several unique features. We have used the technique of aerodynamic acceleration in supersonic seeded beams in order to obtain molecular kinetic energies in the range 1-20 eV. In this energy range, scattering events are impulsive, being free from molecular adsorption on the surface, and surface ionization occurs under nonthermal equilibrium conditions. Thus, in hyperthermal surface ionization (HSI), the kinetic energy is directly converted either into the energy difference between the surface work function and the molecular electron affinity (for negative ions) or into the molecular ionization potential minus the surface work function (for positive ions). Four types of HSI processes were observed. (a) Surface-molecule electron transfer was demonstrated in the 12/diamond system where negative molecular iodine (I2-) ions were produced. (b) Molecule-surface electron transfer was found for the anthracene molecule where positive molecular anthracene ions were generated. (c) Abstractive ionization was detected for N,N-dimethylaniline (DMA) scattered from diamond. A protonated molecular ion was observed. (d) Hyperthermal surface induced dissociative ionization (HSIDI) was observed in 1-iodopropane, benzyl bromide, and many other molecules. In these processes we have observed a large current of negative halogen ions and positive molecular residue ions (ion-pair formation). Hyperthermal surface ionization is characterized by several experimental features such as the following: (a) A very large dependence of the ionization yield on the incident molecular kinetic energy. HSI has an energetic threshold which depends on the molecule, the surface, and the ion. (b) The negative ions were generated on diamond and not on the "technical" metal that served as a support for the diamond crystal. The positive ion yield was larger on this metal support. (c) The effect of the surface temperature on the ionization yield was small. (d) The HSI yield decreased with the beamsurface incident angle. (e) The angular distribution of the scattered ions was shifted toward grazing angles (supraspecular scattering). ( f ) The ions energy distribution was broad, structured, and non-Boltzmann. (g) HSIDI resulted in the generation of ions whose yield correlated with the generation of neutral atoms. The mechanism of hyperthermal surface ionization (HSI) is described in terms of electron-transfer processes. It occurs due to a curve crossing between the neutral scattering interaction potential surface and the ionic interaction potential surface, and a nonunity reneutralization second curve crossing of the scattered molecules or fragments. The HSIDI mass spectra demonstrated a highly nonstatistical m a s spectral fragmentation pattern. This fragmentation pattern was largely different from that of the electron impact ionization mass spectra. It was affected by the fragment electron affinity, by the ionization potential of the molecule or fragment, by surface reactions, and by the reneutralization probabilities. Using iodobutane and dibromopropane isomers we have demonstrated that the HSIDI mass spectra contain important and distinctive isomeric and structural information. We discuss the advantages of HSI as a new efficient ion source and its analytical applications. A HSIDI mass spectrum of a dipeptide such as cyclo-(glycinyltryptophyl) is compared with an electron impact mass spectrum. We also report on low-vacuum experiments using a single-stagevacuum chamber pumped by a diffusion pump or by a rotary pump alone. In these experiments a direct supersonic expansion on the diamond crystal was performed and the ion currents were increased by 3 orders of magnitude up to the 10-5-Arange as compared with the collimated beam-ultrahigh-vacuum experiments.
1. Introduction
Surface ionization is a well-known and established phenomen ~ n . l - ~It can be quantitatively described by the Saha-Langmuir equation which is based on the assumption of thermal equilibrium between the desorbed ions and the emitting surface. Thus the positive ion formation yield is proportional to exp(4 - IP)/kT, where r#~ is the surface work function, IP is the atom or molecular ionization potential, k is the Boltzmann constant, and T is the temperature. The negative ion formation yield is proportional to exp(E, - $ ) / k T where E, is the atomic or molecular electron affinity. The Saha-Langmuir equations show that the ionization yield can be controlled with several limitations and constraints dictated by the surface material, its work function, melting point, and chemical rea~tivity.'-~ An interesting question which is addressed in this paper pertains to the possible surface ionization under thermal nonequilibrium conditions, where the surface is kept at a given relatively low temperature and the molecule carries kinetic energy or internal electronic and/or vibrational-rotational energy. This energy is required to bridge the gap between the molecular ionization potential and the surface work function (positive ions) or the surface work function and the molecular electron affinity (negative ions)! (l).Zandberg, E.Y.;Ionov, N.I . In Surface Ionization; Israel Program for Scientific Translation: Jerusalem, 1971. ( 2 ) Ramsey, N. F. Molecular Beams; Oxford University: London, 1956. (3) Persky, A.; Grecne, E. F.; Kuppermann, A. J . Chem. Phys. 1968,49, 2347.
0022-3654/89/2093-5549$01.50/0
In this respect it should be noted that the kinetic energy seems to be the most important form of energy, as it is also required in order to avoid adsorption which could lead to surface-molecule thermalization. Kinetic energy can be directly converted into the (4 - IP) or (E, - 4) energy differences via surface-molecule or molecule-surface electron-transfer processes. These electrontransfer processes and their many possible outcomes were theoretically dealt with in detail by Gadzuk and H o l l o ~ a y . ~They -~ have denoted them as "surface harpooning", by analogy with the celebrated gas-phase harpoon chemical reactions.* Basically, when a molecule approaches a surface at a given critical distance, R , = e 2 / 4 ( 4- E,), an electron can jump spontaneously from the surface to the molecule. At R , the stabilization energy due to the image potential can overcome the energy difference 4 - E, and a curve crossing from the neutral configuration into an ionic one can occur. The probability for this electron-transfer process can be given by the modified Landau-Zener two-level crossing m0de1.~-~After scattering from the surface the molecule can be reneutralized, but if its kinetic energy is above (6- E,) and the reversed curve crossing probability is not very high, then one can obtain efficient negative ion formation. (The same description (4) Amirav, A.; Danon, A. 'A Method and Apparatus for Producing Ions by Surface Ionization of Energy Rich Molecules and Atoms"; Israel, US., and Great Britain patent applications (1987). (5) Gadzuk, J. W.; Holloway, S. Chem. Phys. Lett. 1985, 114, 314. (6) Holloway, S . ; Gadzuk, J. W. Surf.Sci. 1985, 152, 838. (7) Gadzuk, J. W.; Holloway, S . J . Chem. Phys. 1986, 84, 3502. (8) Herschbach, D. R. Adu. Chem. Phys. 1966, 10, 319.
0 1989 American Chemical Society
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Danon and Amirav
can be applied to molecule-surface electron transfer.) Gadzuk organic molecules can induce alkali-metal ion ejection from a and H ~ l l o w a y ~and - ~ other authors9 have given several distinctive conventional surface ionization detector. The yield of this process was correlated with the ionization potential of the incident organic predictions for the outcome of these "harpoon" processes such as molecules. Electronically excited metastable atoms32and molemolecular vibrational excitation, molecular dissociation, surface c u l e resulted ~ ~ ~ in electron emission upon scattering from metallic chemisorption, electron-hole pair excitation, etc. Surface-molecule surfaces. Electron emission was also observed following hyelectron-transfer processes were observed in ion scattering from perthermal scattering of large molecular clusters.34 metallic surfaces for atomic hydrogen and oxygen'OJ' and subIn this paper we shall describe our findings concerning the sequently for molecular oxygen (positive ions) by Kleyn and various molecular ionization processes at hyperthermal (1-20 eV) co-workers.I2 Until recently these processes were only theoretsurface scattering. We shall describe four processes: (a) negative ically speculated to occur in neutral molecule-semiconductor molecular ion formation or surface-molecule electron transfer; surface systems at near threshold e n e r g i e ~ . ~ - ~ (b) positive molecular ion formation or molecule-surface electron The general field of hyperthermal surface scattering is chartransfer; (c) dissociative ionization processes resulting in both acterized by the discovery of several interesting phenomena such negative and positive fragment ions; (d) abstractive ionization as the following: (a) structured ~cattering;'~ (b) extensive atomsurface energy t r a n ~ f e r ; ' ~(c) J ~molecular vibrational excitati~n;'~J~ where the molecular positive ion attaches a hydrogen atom due to the high molecular proton affinity. We shall describe the (d) molecular chemi~orption;'~*'* (e) surface ~ p u t t e r i n g ; '(f) ~ characteristics of the dissociative ionization which results in a molecular electronic excitation and light emission;20 (g) surface highly "nonstatistical" mass spectra that contain a substantial electron-hole pair formation;21(h) molecular d i s s o c i a t i ~ n ~and ~-~~ amount of structural and isomeric information. Finally, we shall i s ~ m e r i z a t i o n ;(i) ~ ~ion ejection;26 (j)dissociative i o n i z a t i ~ n ; ~ ~ , ~ ~ discuss possible analytical applications and related experiments. (k) molecular decomposition and surface g r o ~ t h . ~ * * ~ ~ 2. Experimental Section Gillen and Bernstein30 have studied the role of molecular vibrational energy on the increased efficiency of a surface ionization The technique of aerodynamic acceleration was used in order detector. Litvak et aL3I have found that, at hyperthermal energies, to obtain molecular kinetic energy in the range of 1-20 eV.35,36 The molecules were seeded in a hydrogen or helium supersonic beam and their partial pressure was controlled by their container's temperature. The ceramic nozzle was a 80 or a 100 pm thin hole (9) Feibelman, P. J. Surf. Sci. 1985,160, 139. Lang, N. D.; Norskov, J. in a small ruby disk (watch jewel) mounted on a 2-mm alumina K. Phys. Scr. 1983, T6,15. Newns, D. M. Surf.Sci. 1986,171,600. Gadzuk, tube. It was described in detail elsewhere.37 J. W. J. Chem. Phys. 1983,79,6341. Gadzuk, J. W.; Norskov, J. K. J. Chem. Phys. 1984, 81, 2828. Holloway, S. J. Vac. Sci. Techno/. 1987, AS, 476. This all-ceramic nozzle could be differentially heated37with Gadzuk, J. W. J. Vac. Sci. Techno/. 1987, AS, 492. a heated volume of cm3. Its ceramic construction and very (10) van Wunnik, J. N. M.; Los, J. Phys. Sci. Scr. 1983, T6, 27. Geersmall heated volume minimized both catalytic and homogeneous lings, J. J. C.; van Amersfoort, P. W.; Kwakman, Tz.; Granneman, E. H. A,; molecular thermal dissociation in the nozzle and allowed us to Los, J.; Gauyacq, J. P. Surf. Sci. 1985, 157, 151. obtain a relatively very high molecular kinetic energy. In this (1 1) Jo, Yang-sun; Schultz, A.; Schuler, T. R.; Rabalais, J. W. J. Phys. nozzle the carrier gas (H2, He) was thermalized during the 100-ps Chem. 1985,89, 2113. Yu, M. L. Phys.Reu. Lett. 1981, 47, 1325. heating flow time, while the polyatomic molecules were not vi(12) Hoachang, P.; Horn, T. C. M.; Kleyn, A. W. Phys. Reu. Lett. 1986, 57, 3035. van den Hoek, P. J.; Horn, T. C. M.; Kleyn, A. W. Surf. Sci. 1988, brationally thermally equilibrated. After the supersonic expansion 198, L335. the molecules attained the carrier gas velocity while losing energy (13) Amirav, A.; Trevor, P.; Cardillo, M. J.; Lim, C.; Tully, J. C. J. Chem. from their vibrational-rotational degrees of freedom. The moPhys. 1987, 87, 1796. lecular kinetic energy could be controlled by changing the backing (14) Kolodney, E.; Amirav. A,; Elber, R.; Gerber, R. B. Chem. Phys. Lett. pressure, the nozzle temperature, or the average mass of the carrier 1985, 213, 303. gas mixture of argon and hydrogen (prepared in an on-line gas (15) Rettner, C. T.; Fabre, F.; Kimman, J.; Auerbach, D. J. Phys. Reu. mixer Matheson R7352T). The nozzle was mounted on a small Lett. 1985, 55, 1904. Rettner, C. T.; Kimman, J.; Fabre, F.; Auerbach, D. chamber that could be heated separately allowing us to control J.; Morawitz, H. Surf. Sci. 1987, 192, 107. the partial pressure of nonvolatile organic molecules. (16) Danon, A.; Amirav, A.; Silberstein, J.; Salman, Y.; Levine, R. D. J. Phys. Chem. 1989, 93, 49. The molecular beam was skimmed and collimated through two (17) Balmh, M.; Cardillo, M. J.; Miller, D. R.; Stickney, R. E. Surf.Sci. differentially pumped chambers into an ultrahigh-vacuum (UHV) 1974, 46, 358. Torr). The accelerated beam chamber (base pressure 5 X (18) Ceyer, S. T.; Beckerle, J. D.; Lee, M. B.; Tang, S . L.; Yang, Q.Y.; was either square-wave-modulated for phase sensitive detection Hines, M. A. J. Vac. Sci. Techno/. 1987, AS, 501. Rettner, C. T.; Delouise, or mechanically chopped for kinetic energy measurements by using L.A.; Auerbach, D. J. J. Chem. Phys. 1986,85, 1131. Lee, J.; Madix, R. a time-of-flight (TOF) technique. In the UHV chamber, the beam J.; Schlaegel, J. E.; Auerbach, D. J. Surf.Sci. 1984, 143, 626. collided with a single-crystal diamond (1 11) surface. The diamond (19) Liu, S. M.; Rodgers, W. E.; Knuth, E. L. J. Chem. Phys. 1974,61, 902. as purchased as a "macle" flat crystal and its dimensions are 13 (20) Bottari, F. J.; Greene, E. F. J. Phys. Chem. 1984, 88, 4238. X 10 X 2.1 mm3. It was carefully mechanically polished and (21) Amirav. A.: Cardillo. M. J. Phvs. Reu. Lett. 1986.57.2299. Amirav. chemically treated by a hot (250 "C) KN03/H2S04solution and A.;'Lambert, W. R.; Cardillo, M. J.; Trevor, P. L.; Luke, P. N.; Haller, E: then rinsed with triple distilled water. The diamond was mounted E. J. Appl. Phys. 1986, 59, 2213. on a heatable tantalum mount with its grazing angle unperturbed. (22) (a) Kolodney, E.; Amirav, A. J. Chem. Phys. 1983, 79, 4648. (b) It was clamped to its mount by two 0.1-mm molybdenum foil strips Kolodney, E.; Amirav, A. Dynamics on Surfaces, Pullman, B., Jortner, J., and it had an exposed area of 10 X 10 mm2. The beam was Nitzan, A., Gerber, R. B., Eds.; Reidel: Dordrecht, 1984; pp 231-243. (c) Kolodney. E.; Amirav, A.; Elber, R.; Gerber, R. B. Chem. Phys. Lett. 1984, scattered from the surface center and its dimensions at the surface 111, 366. were 3.5 X 1.2 mm2. The diamond was heated in the vacuum (23) Gerber, R. B.; Amirav, A. J. Phys. Chem. 1986, 90, 4483. and annealed at 850 O C to give the helium diffraction pattern, (24) Danon, A,; Kolodney, E.; Amirav, A. Surf.Sci. 1988, 193, 132. (25) Prada-Silva, G.; Loffler, D.; Halpern, B. L.; Haller, G. L.; Fenn, J. B. Surf. Sci. 1979, 83, 454. (26) Amirav, A.; Cardillo, M. J. Surf.Sci. 1988, 198, 192. (27) Danon, A.; Amirav, A. J. Chem. Phys. 1987, 86, 4706. (28) de la Mora, J. F. J. Chem. Phys. 1985, 82, 3453. (29) Connolly, M. S.; Greene, E. F.; Gupta, C.; Marzuk, P.; Morton, T. H.; Parks, C.; Staker, G. J. Phys. Chem. 1981, 85, 235. (30) Gillen, K. T.; Bernstein, R. B. Chem. Phys. Lett. 1970, 5, 275. (31) Litvak, H. E.; Gersh, M. E.; Bernstein, R. B. Chem. Phys. Lett. 1975, 36, 145.
(32) Johnson, P. D.; Delchar, T. A. Surf. Sci. 1978, 77, 400. Hagstrum, H. D. Phys. Reu. Lett. 1979, 43, 1050. (33) Sne, 0.; Cheshnovsky, 0. Chem. Phys. Lett. 1986, 130, 53. (34) Even, U.; de Lange, P.; Jonkman, H . ; Kommandeur, J. Phys. Reu. Lett. 1986, 56, 965. (35) (a) Pauly, H.; Toennies, J. P. Methods of Experimental Physics. Bederson, B., Fite, W., Eds.;Academic Press: New York, 1968; Vol. 7A, p 227. (b) Anderson, J. B.; Andrw, R. P.; Fenn, J. B. Adu. Chem. Phys. 1965, 10, 215. (36) Kolodney, E.; Amirav, A. Chem. Phys. 1983, 82, 269. (37) Danon, A,; Amirav, A. Rev. Sci. Instrum. 1987, 58, 1724.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5551
Surface Ionization of Molecules I
I
12/ D I A M O N D
Zl t z 91
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Figure 1. Mass spectrum of negative ions formed after I,/diamond scattering at 8.8 eV iodine kinetic energy. The surface temperature was 380 “C and the incident angle was 20°. The ceramic nozzle temperature was 220 O C and the hydrogen backing pressure was 1040 Torr. The iodine sample temperature was 40 O C (similar spectrum was obtained by using a room temperature nozzle).
as shown in ref 24. This helium diffraction and similar hydrogen diffraction3*(similar to that previously reported39) served as an on-line check of the diamond’s cleanliness while scattering a hydrogen-seeded beam. The specular reflection was somewhat broader than our angular resolution reflecting some surface imperfections and we also suspect that the surface may be at least partially covered with hydrogen. A channelled ionization gauge served as a flux detector at 90’ to the molecular beam axis for the diffraction studies. Two quadrupole mass spectrometer (QMS) heads (UTI 1OOC) served as detectors. One was mounted in line with the beam for kinetic energy measurements and it also helped to identify and control the seeded beam composition. The second QMS head was mounted at 45O to the molecular beam axis and it served as a detector for the scattered beam. The detector-surface distance was either 2.5 cm for the ionization studies at 38 cm behind a liquid nitrogen trap for a higher angular resolution and scattered beam time of flight measurements. This QMS head was modified for external ion collection and negative ion detection. (Reversed channeltron voltage biasing.) This collection lens also served for a fly-through operation without secondary collisions. The positive ion mass assignment was calibrated with the electron impact ionizer, but the negative ion mass spectra could not be calibrated in this way and the negative ions’ mass assignments are accurate only within f l atomic mass unit (amu). Absolute ionization probability calibration was performed by measurements of the current to ground from the surface holder or from a nearby Faraday cup. The total molecular flux to the UHV chamber was measured by using an effusive beam as a calibrated flux source. The estimated uncertainty factor in the absolute calibration was between 2 and 3. The relative precision was 20% with current and relative molecular flux measurements. The molecular relative fluxes were measured by the rise of the molecular partial pressure at a hindered Q M S head. Angular distributions were obtained by rotating the surface so that the incident beamsurface angle was scanned, while the beam detector angle was fixed at 45O (QMS)or 90° (Auger). An Auger electron spectrometer (VG CLAM 100) served for ion energy analysis and ion angular distribution measurements. This detector was a hemispherical analyzer and its entrance lens was at 50 mm from the surface and 90’ to the beam axis. The ion energy resolution was better then 0.2 eV and with simple external voltage modifications it could also analyze positive ions. Another set of experiments was performed in a single vacuum chamber with a direct expansion of an unskimmed beam on a surface a n d current measurements, which will be described in section 12. 3. Surface-Molecule Electron Transfer: IJDiamond Scattering at 1-12 eV We have observed the generation of a large current of negatively A), upon scattering of hyperthermal charged ions ( I > (38) Kolodney, E.; Amirav, A. Surf. Sci. 1985, 155, 715. (39) Vidali, G.; Frankel, D. R.Phys. Rev. 1983.827, 2480.
Ek (ev)
Figure 2. Absolute 12 formation yield versus the average molecular iodine incident kinetic energy. (a) ( 0 )represents measurements using the ceramic nozzle at room temperature and hydrogen/argon gas mixture as a carrier gas at variable argon concentrations in the range 0 - 1 2 1 mixture. (b) (+) represents measurements using the ceramic nozzle at variable nozzle temperatures in the range of 30-300 O C , and pure hydrogen as a carrier gas. The nozzle backing pressure was 1040 Torr. Practically identical results were obtained by using helium as a carrier gas or a stainless steel conical nozzle. The diamond surface temperature was 450 O C and the incident angle was 3 5 O .
molecular iodine on diamond” Figure 1 shows the mass spectrum of these ions. The dominant feature is at mass 254 of the negative molecular ion. An additional small peak ( 2%) at 127 amu is of atomic iodine. Based on Figure 1 we conclude that (a) surface-molecule electron transfer occurs a t hyperthermal energy scattering; (b) in the case of molecular iodine the generated ions mostly do not dissociate. Upon heating the nozzle to -650 O C the mass spectrum becomes totally dominated by negative atomic iodine ions due to the thermal dissociation of molecular iodine in the nozzle. In Figure 2 we plot the kinetic energy dependence of the absolute production yield of negatively charged molecular iodine versus its kinetic energy. A practical threshold of 2.8 eV is observed. We note that in our TOF measurements the molecular beam was chopped and the generated 1,- were detected. The obtained TOF was corrected for the ions’ TOF (surface-detector) by ion energy extrapolation to infinity. Thus the observed threshold is inherently corrected to the incident velocity distribution width. This threshold is somewhat lower than the expected 3.5-eV threshold energy based on the difference between the surface work function (6.1 eV) and the molecular electron affinity (2.6 eV). Perhaps this lower threshold is due to surface impurities or imperfections that lower the diamond work function to 5.4 eV. W e attempted but could not measure any positive atomic or molecular iodine and the positive ions mass spectrum exhibits mostly a trace amount of alkali-metal (K+, Na+) ions ejected from the surface, as is described in detail for other “clean” surfaces.26 When the surface height or position was scanned, we found that the production yield of negative ions dropped to zero on the technical “dirty” tantalum and molybdenum metals that served as a holding support and clamps for the diamond. We shall discuss this aspect in detail in section 9. In Figure 3 we show the kinetic energy distributions of the negative ions as measured by an Auger hemispherical energy analyzer. The kinetic energy distributions are broad and some ions contain almost all the possible initial kinetic energy minus the difference between the work function and the molecular electron affinity. However, the width of the ion kinetic energy
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(40) Danon, A.; Amirav, A. Phys. Rev. Lett. 1988, 61, 2961.
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The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 I /
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Danon and Amirav
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Figure 3. Kinetic energy distributions of the negative molecular iodine formed following 12/diamondscattering at 8.2 eV kinetic energy. The molecular beam-detector angle was fixed at 90°. The outgoing scattering angles are indicated, and the incident angles are 90' - Bo. i
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Figure 5. Absolute anthracene' formation yield versus the average molecular anthracene incident kinetic energy. Kinetic energy was controlled by the average mass of the hydrogen/argon gas mixture and at high energies by the nozzle temperature which was differentially heated up to 500 O C . The anthracene sample temperature was 110 OC and the nozzle backing pressure was 1000 Torr.
-
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Figure 4. Mass spectrum of positive ions formed after anthracene/diamond scattering at 8.5 eV anthracene kinetic energy. Similar spectra were obtained in all the kinetic energy range 3-15 eV. The insert magnifies the molecular parent ions spectral range.
distribution functions is large, reflecting the involvement of the surface in kineticsurface or kinetic-intemal vibrational-rotational energy-transfer processes.I6 We also note that the energy distribution is bimodal, as will be discussed in section 9. As expected from the existence of the image potential, the ions were scattered at supraspecular angles. These angular shifts toward grazing of the surface were especially pronounced for ions having a lower kinetic energy.
4. Molecule-Surface Electron Transfer: Anthracene/Diamond Scattering at 1-15 eV So far we have demonstrated the existence of surface-molecule electron transfer which, according to theoretical consideration, is facilitated by the image potential near the surface. When considering a molecule with a low ionization potential, such as anthracene (IP = 7.55 eV4'), another possibility exists, namely, electron transfer from the molecule to the surface. In this case the image potential helps to bridge the energy gap between the molecular ionization potential and the surface work function. In similarity to the production of Iz-, we expect that inefficient reneutralization would result in the production of positive molecular ions. When we scattered the polyatomic anthracene molecules on the diamond at hyperthermal energies, we observed a current of positive ions. Figure 4 shows the mass spectrum of these ions. This mass spectrum is clearly dominated by the undissociated molecular ion (A+) (including the naturally abundant 13Canthracene). The other minor peak observed a t mass 39 is of potassium due to ion ejection of K impurity, as is described in detail elsewhere.26 Figure 4 demonstrates the existence of molecule-surface electron transfer. Figure 5 shows the kinetic (41) Weast, R. C., Ed. Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981-82.
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energy dependence of the absolute production yield of positive molecular anthracene ions. The experimental trend is similar in appearance to that of 1, as shown in Figure 2. A practical threshold of 3 eV is observed and the yield increases by 5 orders of magnitude when the kinetic energy is increased up to 15 eV. As in Figure 2 we note that the experimental points near the threshold are inherently corrected to the velocity distribution width, as the T O F measurements were performed while detecting the anthracene ions (TOF of those molecules which were ionized alone). The observed threshold is higher than the 1.5-eV thermodynamic threshold based on the difference between the anthracene ionization potential of 7.55 eV and the diamond work function of 6.1 eV.42 As reported herein this finding can be at least partially rationalized if we assume a lower diamond work function due to surface impurities or imperfections as well as a lower real threshold. The crucial role of kinetic energy in the promotion of molecule-surface electron transfer is clearly demonstrated in Figure 5 . We note that, unlike the case of Iz-, the production yield of A+ is maximized on the technical "dirty" molybdenum metal foil, which served as a holding clamp of the diamond. The A+ yield was higher by a factor of -20 on this molybdenum foil than on the single-crystal diamond. There is some probability that the near-threshold results might be affected to some extent by secondary collisions or by the geometrical tail of the beam cross section. At 15 eV kinetic energy the A+ yield reached the level of 1% on the molybdenum foil, while it was 5 X lo4 on the diamond at 14.4 eV. Under the experimental conditions described we failed to observe any negative ions as the result of the anthracene hyperthermal diamond or molybdenum scattering.
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5. Abstractive Ionization: Diamond-N,N-Dimethylaniline Proton Transfer One known ionization mechanism occurs via proton transfer to an organic base molecule and is stimulated by the large molecular proton affinity (- 10 eV) of a base such as DMA. This type of abstractive ionization is known in field i ~ n i z a t i o n in ,~~ chemical ionization, and even in thermal surface i o n i z a t i ~ n . ~ , ~ ~ (42) Neuterger, M. Handbook of Electronic Material; IFI/Plenum: New York, 1971. (43) Beckey, H.D. Principles of Field Ionization and Field Desorption Mass Spectrometry; Pergamon: New York, 1977. (44) Fujii, T.; Kitai, T. Int. J. Mass. Spectrom. Ion Processes 1986, 71, 129.
The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
Surface Ionization of Molecules
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ELECTRON IMPACT 70 e V
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Figure 6. Positive ion mass spectra of N,N-dimethylaniline: (a) upper trace is of electron impact ionization at 70 eV electron energy; (b) middle trace is of hyperthermal surface ionization (HSI) from the diamond surface at 5 eV kinetic energy; (c) lower trace is HSI from the technical molybdenum foil that clamped the diamond crystal to its holder.
At hyperthermal energies the scattering event is impulsive and lasts on a subpicosecond time scale. Nevertheless, we have observed abstractive ionization as demonstrated in Figure 6 . Figure 6 shows the mass spectra of N,N-dimethylaniline (DMA). The upper trace is a conventional electron impact mass spectrum at 70-eV electron energy of surface-scattered molecules. We observe the molecular parent ion and M - 1 of electron-induced hydrogen dissociation. The lowest trace shows the mass spectrum of ions formed after DMA scattering from the technical molybdenum surface. (The electron impact ionizer was turned off and the surface was biased by +10 V). The largest feature is of the molecular DMA+ ion but we also observe (DMA - 1)' ions due to dissociative ionization that will be described in the next section. In the middle trace we show the resulting mass spectrum following DMA/diamond scattering. It is clearly observed that the mass spectrum is dominated by (DMA 1)' and 13C(DMA 1)' ions reflecting abstractive ionization which is driven by the large proton affinity of DMA (- 10 eV). Another important feature is of (DMA - 1)+ ions which is due to dissociative ionization into (DMA - 1)' and H-. Although the (DMA - 1)+ feature is weaker than (DMA + l)+, we observed a larger Hcurrent in the negative ion measurements than in the total yield of positive ions (see section 6 ) . We note that the diamond is believed to be at least partially covered by hydrogen.46 Hydrogen, which served as our carrier gas, could also be supplied by the dissociative ionization or scattering processes. Similar abstractive ionization was also observed by us in the scattering of DABCO (C6HI2N2)from the diamond. Finally, we note that in DMA scattering the positive ion yield was larger on the molybdenum foil, while H- production could be observed only on the diamond.
110
2 IO
MASS(au) Figure 7. Positive (A) and negative (B) ion mass analysis of I-iodopropane ionized due to its high kinetic energy (7 eV) scattering from diamond (1 1 I). The surface temperature was 450 OC. The surface was biased at -20 V and +20 V for the negative and positive ion detection, respectively.
6. Dissociative Ionization: 1-Iodopropane/Diamond Scattering at 1-10 eV One of the most important features of molecular ionization in hyperthermal surface scattering is the observed unique dissociative
ionization pattern. In Figure 7 we show the positive and negative ions' mass spectra obtained from 1-iodopropane scattered at 7 eV from the diamond surface." In both spectra, the parent ion mass (170) is missing and mostly I- or propyl+ are observed, manifesting hyperthermal energy surface induced dissociative ionization (HSIDI). The negative ions mass spectrum contains mass 127 (I-) alone while the positive ions mass spectrum also contains mass 57 of butyl positive ions. We have identified its origin as small amounts of tert-butyl iodide (
